Lineage-splitting and extinction have taken place often throughout the
history of life  they are not rare,
one-time only events. That means that if scientists have a lot of information
about a clade, they can calculate its rate
of extinction or the rate of lineage-splitting (known as the rate
of diversification).

The balance between a clade's rates of diversification and extinction
determines whether or not the entire clade will go extinct. If, for
example, your school's yodeling club is losing members faster than it
can bring them in, it won't
be around for long.

If this process continues, the yodeling club will be decimated in 15 months.

The same processes operate on a clade's diversity: if extinction happens more frequently than lineage-splitting,
that entire clade will go extinct eventually. For example, trilobites and ammonites had high rates of both
diversification and extinction.

That means that looking back in the fossil record, we see many different trilobite and ammonite lineages,
but few that lasted for long periods of geologic time. And of course, for both of these clades, in the long
run, extinction happened more frequently than lineage-splitting, with the result that both are now entirely extinct.

Understanding rates of extinction and diversification also gives us a new way to look at diversity. For example,
a clade might be large because it is generating new members (high rate of diversification), or it might be large
because it rarely loses members (low rate of extinction).